Up to now three different DNA binding modes have been proposed. One is the α-helix represented by the helix-turn-helix structure of prokaryotic repressors and eukaryotic homoeotic gene products. Another is the antiparallel β-sheet found in G5BP and DBP II. The other is the Zn-finger originally suggested for TF IIIA.
The fourth quite new mode of DNA binding was proposed for histones H1 and H2B and then generally found in many gene reguratoly proteins. This is a repeat of β-turns composed of Ser(Thr)-Pro-X-X sequences.
